Human cells obtained either from individuals with some medical condition or from apparently normal individuals or populations will be cultured and studied cytogenetically, immunologically, biochemically, and with respect to growth in vitro. In addition, careful clinical correlations with laboratory investigation will be made by use of pedigrees and medical records. Some studies will be clinical investigation, and others will be basic laboratory experimentation using human cells. Specific projects include the cause of chromosomal breakage in Bloom's syndrome; the comparison of DNA synthesis in different cell types; the antigenic variation in cultured human fibroblasts; and the use of marker or structurally abnormal chromosomes to assign genes to specific human autosomes.